Inkjet inks typically include a pigment or a dye as a colorant in an ink vehicle, such as an aqueous ink vehicle. The decision of whether to use a dye or a pigment in the inkjet ink depends on the particular printing application because each of the colorants provides specific advantages and disadvantages. Dyes are usually water soluble, which makes formulating dye-based inkjet inks relatively easy. However, images printed with the dye-based inkjet inks commonly smear or rub when contacted or exposed to water or moisture. Images printed with dye-based inkjet inks also are not stable to visible light, ultraviolet light, or sunlight. In contrast, pigment-based inkjet inks produce printed images that are more waterfast and lightfast than those produced with dye-based inkjet inks. However, the pigment does not penetrate into the print medium and remains on its surface. Therefore, the printed image has a low resistance to wet smudge and a low highlighter resistance. In addition, achieving and maintaining a uniform dispersion of the pigment is difficult because the pigment is not readily soluble in the ink vehicle. To stably disperse the pigment, dispersing agents, such as polymeric dispersants or surfactants, have been added to the inkjet ink. In addition, surfactants have been adsorbed or coated onto the pigment to improve dispersibility. However, adding polymeric dispersants or surfactants increases the viscosity and affects the printability of the inkjet ink. Furthermore, these pigment-based inkjet inks do not produce durable images when printed on a porous print medium.
To improve the dispersibility of the pigment in the ink vehicle, chemical modifications to the pigments have also been suggested. Pigments have been chemically modified by attaching an organic functional group, such as a small molecule or a polymer, to a surface of the pigment. One problem with inkjet inks including modified pigments, particularly inkjet inks having small molecules attached to their surfaces, is that they do not produce durable images when printed on porous print media. Therefore, to provide increased durability on these print media, polymeric additives have been used in the inkjet ink. However, use of these polymeric additives produce an increase in the viscosity of the inkjet ink, which decreases its printability, optical density (“OD”) and resistance to wet smudge.
It would be desirable to provide a pigment-based inkjet ink that exhibits improved wet smudge durability without decreasing OD when printed on a porous print medium.